1920 .] 
Departmental Report. 
225 
in colour than the bulk of the overlying tuffs, though not so dark as the 
friable tuff near the base of the B beds. 
The following analyses made in the Dominion Laboratory show the 
material to be very similar 
to the 
“ wilsonite ” 
tuff of Waihi. 
(1) 
(2) 
(3) 
(D 
(5) 
(6) 
(D 
Silica (SiOo) 
67-83 
67-82 
69-80 
48-83 
70*06 
60-45 
72-30 
Titanium dioxide (TiO<>) 
0'43 
0-37 
n.d. 
0-28 
0-31 
0-38 
0-12 
Alumina (A1 2 0 3 ) . . 
14-68 
17-21 
14-37 
21-06 
14-66 
13-85 
12-50 
Iron oxide (Fe 2 0 3 ) 
4-79 
0-52 
4-74 
1-30 
1-53 
3-69 
2-59 
Manganese oxide (MnO) 
0-06 
0-05 
n.d 
n.d. 
n.d. 
n.d. 
0-03 
Lime (CaO) 
2-81 
1-83 
2-05 
1-29 
1-42 
1-82 
1-35 
Magnesia (MgO) 
0-69 
0-55 
0-4D 
0-30 
0-20 
0-13 
o-io 
Soda (Na 2 0) 
3-46 
3-02 
2-39 
0-62 
3-30 
3-44 
3-25 
Potash (K 2 0) 
2-90 
3-28 
3-76 
1-13 
3-78 
3-48 
3-58 
Phosphorus pentoxide (P 2 0 5 ) 
n.d. 
n.d. 
n.d. 
n.d. 
n.d. 
n.d. 
0-31 
Combined water and organic 
matter. . 
Water lost at 100° C. 
1-87 
0-66 
3-25 
1-88 
j 4-60 
f 10-37 
| 15-21 
3-37 
1-21 
3-05 
1-38 
3-54 
0-46 
100T8 
99-78 
100-11 
100-39 
99-84 
100-67 
100-13 
(1.) Typical specimen of tuff from C beds, western bank of river. 
(2.) Typical specimen of tuff from C beds, bottom of gorge. 
(3.) Typical specimen of tuff from the upper portion of B beds, eastern side of gorge. 
Collected bj^ the writer in 1918. The iron oxide of the analysis includes 
1-26 per cent, of ferrous oxide. 
(4.) Much-weathered tuff near top of cliff, eastern side. A sample of tuff from the • 
A beds decomposed to material having the composition of a clay. 
(5.) Soft pumiceous sandstone near base of B beds, eastern side of river. 
(6.) Friable very-dark-coloured tuff overlying No. 4, eastern side of river. 
(7.) “ Wilsonite ” tuff near Waikino, Waihi (N.Z. Geol. Surv. Bull. No. 16, p. 73, 1913). 
The iron oxide of the analysis includes 047 per cent, of ferrous oxide. 
Samples 1, 2, 4, 5, and 6 were collected by the Public Works Engineer. 
The cliffs of the gorge at the dam-site are formed of great vertical 
columns of rock, in cross-section roughly five- or six-sided, and separated 
from one another by joint-planes. In the excavations these joints appear 
usually as mere cracks, but a few are up to J in. across. Some of the larger 
are filled with a dark buttery material. The great majority of the cracks 
strike within 10° of the bearings 67°, 147°, and 177° (true), suggesting that 
they are the planes of a columnar joint-system and not connected with 
fault movements. Decisive evidence on this point was observed in several 
of the excavations, in that the cracks sever pieces of pumice without 
displacement save that produced by the slight opening along the joint- 
plane. A curious feature is that on the western side of the river the walls 
of the joints are closely pressed together, whereas on the eastern side many 
of the joints form readily-observable open cracks. 
A drive projected from the bottom of a shaft sunk on the western side- 
of the river passes under the stream to the eastern side. There it penetrated 
a crack with a flow of water greater than could be kept down by bailing or 
by a water-jet, the only means at the disposal of the engineer. A drive from 
a shaft on the eastern side of the river approaches within 2J ft. of the end 
of the drive from the west. The writer examined this drive. The tuffs are 
relatively porous rocks, and, except in wet weather, surface water does not 
flow in the beds of the smaller streams. But the pores in the rock must be 
very small and percolation very slow, since the tuff partition separating the 
end of the drives, and but 2J ft. thick, resists water under a head of 60 ft. 
without perceptible seepage. 
The Government Analyst ground portions of the analysed samples 
Nos. 1, 2, and 6 to pass a 60-mesh sieve. Twenty grams of the part rejected 
